Background
Developmental dysplasia of the hip (DDH) is one of the most common musculoskeletal disorders in children, and some patients may develop hip osteoarthritis in early adulthood and already require joint replacement [
1]. The treatment of DDH is determined by the age at initial diagnosis, degree of hip dislocation, and initial therapeutic effects. Patients with DDH between 6 and 24 months or with failed Pavlik harness treatment within 6 months are treated with either conservative or surgical treatment modalities, such as closed reduction (CR) with spica casting and femoral osteotomies. CR and spica casting are the most utilized conservative treatment [
2,
3].
The patient needs an intraoperative X-ray arthrography to evaluate whether CR is successful and to detect abnormal soft tissue structures in the hip, such as hypertrophied acetabular cartilage, narrowed capsule, and labral inversion [
4,
5]. A plain pelvic film was typically used for follow-up after CR. However, X-ray films cannot evaluate the alignment of the acetabulum and the femoral head accurately nor observe the changes in soft tissue structures of the hip directly [
6]. Magnetic resonance imaging (MRI) has no radiation and can clearly show the soft tissue and osseous structures of the hip joint, and its use in DDH detection and follow-up has been increasing [
7‐
9].
Currently, it is debated whether the abnormal soft tissue structures in the hip of DDH patients can hinder CR and affect the outcome. Renshaw et al. [
10] found that “false reduction”, where reduction was achieved immediately after CR but eventually the hip joint became unstable due to obstruction of soft tissue structures, can occur in some patients. However, Druschel et al. [
11] believed that abnormalities of soft tissue structures did not affect the success of CR.
Initially, it is often impossible to obtain a concentric reduction in the affected hip after CR. Complete relocation is achieved when the femoral head reaches the bottom of the acetabulum [
12]. Some authors believe that complete relocation may increase the risk of femoral head necrosis, which may be caused by increased pressure in the hip joint after reduction or due to damage to the blood supply of the femoral head when performing CR [
13]. In addition, risk factors for residual acetabular dysplasia (RAD) after CR are also being studied extensively. Some authors believe that the cartilaginous/osseous acetabular index, cartilaginous/osseous central-edge angle, the shape of the acetabular load area, and the abnormal high signal intensity on T2WI of acetabular cartilage are risk factors for RAD [
14,
15].
In this study, MRI was used to observe changes in the soft tissue structure of the hip joints of DDH patients undergoing CR. This study also aimed to explore whether complete relocation is a risk for femoral head necrosis, and to identify the risk factors of RAD after CR.
Discussion
The study found that after 4–6 months of CR and spica casting, the ligamentum teres, the transverse ligament, the pulvinar, and the LCC in the affected hip joint gradually returned to normal shape, and 61.8% of the femoral head had complete relocation. The patients who followed up for more than 18 months revealed that the rate of the femoral head necrosis caused by CR was about 8.6, and 48.3% of the OAI returned to normal. Hypertrophy and partial inversion of the LCC 4–6 months after CR were risk factors for RAD, while hypertrophic ligaments, pulvinar, and femoral head complete relocation had nothing to do with RAD.
In this study, LCC was classified based on MRI images. The results show that this classification has good consistency, high reliability, and repeatability, and can be used in everyday work. Before CR, the labra were everted, and during CR, the labra were caught up in the hip joint with the reduction of the femoral head, resulting in labral hypertrophy and inversion. Therefore, most of the LCC cases were type 4 and a few were type 3 immediately after CR. With the gradual inward displacement of the femoral head after reduction and complete relocation, the LCC also gradually changes shape, the labra gradually grow outward with acetabular cartilage thinning, and it returned to the normal shape. For type 2 LCC, the everted labrum is more common in patients with subluxation of the hip joint before CR. The labrum is everted because of the outward and upward displacement of the femoral head, which is rarely seen after CR.
There are many studies on soft tissue structures in and around the hip joint, which hinder CR in DDH patients [
22‐
24]. Studer et al. [
22] observed that hypertrophy of the ligamentum teres, transverse ligaments, pulvinar, joint capsule, inverted labrum, and acetabular cartilage hypertrophy were important factors that hindered CR. Rosenbaum et al. [
25] concluded that labral hypertrophy and inversion, and hypertrophy of the pulvinar, ligamentum teres, and transverse ligaments were the main reasons hindering CR. By arthrography and MRI, Hattori [
24] and Kim [
26] found that obvious soft tissue insertion in the hip joint, widening of the medial contrast cistern, and LCC hypertrophy would increase the probability of CR failure, thus increasing the need for open surgery, even if surgical treatment cannot achieve a good prognosis. Yuan et al. [
27] found that poor delineation of the labrum and acetabular surface during arthrogram predicted failure of CR in children with DDH, and medial dye pool distance ≥6 mm significantly increased the risk of CR failure. However, the studies by Severin [
12] and Aoki [
28] have shown that the inverted labrum can be gradually shaped and returned to a normal shape after CR without affecting the final acetabular-head alignment. Walter et al. [
29] believed that the hypertrophy of soft tissues in the hip joint does not lead to CR failure. The failure is due to the mismatch between the femoral head and the acetabulum. Lü et al. [
30] found that if the LCC was thin, most hips could be successfully reduced and achieve complete relocation, while patients with thick LCC would prevent reduction of the femoral head. In our study, the incidence of hypertrophy of the pulvinar, ligamentum teres, and transverse ligaments was high before and at the time of CR, but these structures gradually returned to normal. Therefore, the authors believe that the abnormal soft tissue structures of the affected hip joints at reduction have no significant effect on the ultimate outcome, and it seems that it is not necessary to deal with these structures at the time of reduction. The CAHI of the affected hips increased gradually after CR, suggesting that CR increases the stress between the femoral head and the acetabulum and the cartilaginous acetabulum develops.
This study found that at the second time of replacing spica casts (4–6 months after CR), most femoral heads can achieve complete relocation, meaning that the hip joint achieved concentric reduction. One of the most serious complications of CR is secondary femoral head necrosis. The cause of necrosis is unknown, but may be related to the interruption of blood supply to the femoral head, excessive abduction of the hip joint, and increased stress on the femoral head. It is suggested that the rate of femoral head necrosis after CR in patients with DDH varies between 0 and 67% [
13,
31]. This study found that the rate of femoral head necrosis with 18 months or more of follow-up after CR was 8.6%, which was acceptable and were lower than the multicenter study of Li et al. [
3]. It was shown that with the improvement of orthopedic surgeons’ understanding of the “safe zone” when performing hip abduction, CR and spica casting proved to be a safe and effective treatment for 6–24-month-old DDH patients.
In this study, it was found that partial inversion of the labrum at 4–6 months after CR was a risk factor for RAD, while complete relocation of the femoral head does not influence the development of osseous acetabulum. Some risk factors of RAD have been found [
32]; however, these factors are still controversial, and there is a lack of in-depth research on the causes of abnormal signals and parameters of acetabular cartilage. We have conducted a study on the application of T2 mapping combined with CUBE [
33], which found that the T2 values of acetabular and femoral head cartilage in patients with inverted labra were higher than those in patients without labra inversion, and the more serious the labral inversion, the higher the T2 value, suggesting that the acetabular cartilage in patients with labral inversion was mostly made of hyaline cartilage with increased free water content, and it could not be mineralized in time. Combined with this study, we believe that labral inversion can hinder the normal ossification of the acetabular cartilage, resulting in RAD after reduction. The complete relocation of the femoral head suggests that CR can achieve a concentric reduction of the hip in patients with DDH, but if the labral inversion persists, it will hinder the normal development of acetabular cartilage.
This study has some limitations. First, the sample size in the RAD group and the normal acetabular group included in this study was small, and the diagnostic efficiency was insufficient. It is necessary to further increase the number of patients in each group in the future. Second, the follow-up time of the patients after CR was uneven and relatively short, without follow-up until the patients grew up, and the outcome of the condition of the patients was unknown. The follow-up time of the normal acetabular group is longer than that of the RAD group, so there may be some patients in the RAD whose OAI returned to normal with time. Therefore, it is necessary to continue long-term follow-up in these patients to observe the outcome. Lastly, the use of MRI repeatedly in young children is impractical in the clinical setting because of compliance, need for anesthesia, and expenses; therefore, it is very difficult to perform MRI examinations universally.
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